There are two ways one can group all the strategies for Sudoku: by difficulty and by family. Difficulty is rather subjective but necessary, for example, when selecting the order in which strategies are tried in the solver. Some strategies will always be easier for some people to spot than others, but I believe I have chosen an ordering which is not too controversial. So the main documentation has a side menu organised by difficulty. There is also an article on Brute Force vs Logic.

- Getting Started
- Naked Pairs
- Naked Triples
- Naked Quads
- Hidden Pairs
- Hidden Triples
- Hidden Quads
- Pointing Pairs
- Box/Line Intersection

- Almost Locked Sets
- Finned X-Wing
- Finned Swordfish
- Franken Swordfish
- Pattern Overlay
- Aligned Pair Exclusion
~~Empty Rectangles~~- Exocet
- Double Exocet
- Sue-de-Coq
- Death Blossom
- Bowman's Bingo
~~Multivalue X-Wing~~~~Guardians~~

With chaining strategies, there is definitely a theme going through them. This theme is all about bi-value (only two candidates left in the same cell) and bi-location (only two occurrences of a particular candidate left in the same unit) pairs and the incredible number of deductions one can make from them. You will find, if you read through this group, that earlier strategies become part of a more general theory as the theme develops. Thus, for example, Remote Pairs are a subset of XY-Chains; that is, XY-Chains is a more general approach of which Remote Pairs are a specific instance. Do read the introductory articles Introducing Chains and Links and Weak and Strong Links.

Exotic strategies do overlap with chaining ones, but they have a peculiar flavour of their own and some wonderful, if obscure, logic. They are definitely worth presenting as a demonstration of people's ingenuity but you will only need to have recourse to them on the extreme puzzles.

There are naturally special strategies for Jigsaw and Killers because of their differences. These are now included for the first time on this site.

This strategy list is by no means complete. Many strategies can be further extended and we do not have a complete theory of all Sudoku puzzles. If you are interested in the concepts behind creation and grading, there is a PDF document here called Sudoku Creation and Grading. With the community's help I hope to extend the documentation here.

For those people wondering why "Escargot" cannot be solved by the solver, there is an article on this special Sudoku here. This is an early 'ultimate puzzle' but this crown has been usurped by the puzzle created by Arto Inkala, which is also in the example list.

I've been looking at a new idea for measuring the difficulty of very hard puzzles - ones that can't use the standard scoring because they don't complete.

Exotic strategies do overlap with chaining ones, but they have a peculiar flavour of their own and some wonderful, if obscure, logic. They are definitely worth presenting as a demonstration of people's ingenuity but you will only need to have recourse to them on the extreme puzzles.

There are naturally special strategies for Jigsaw and Killers because of their differences. These are now included for the first time on this site.

This strategy list is by no means complete. Many strategies can be further extended and we do not have a complete theory of all Sudoku puzzles. If you are interested in the concepts behind creation and grading, there is a PDF document here called Sudoku Creation and Grading. With the community's help I hope to extend the documentation here.

For those people wondering why "Escargot" cannot be solved by the solver, there is an article on this special Sudoku here. This is an early 'ultimate puzzle' but this crown has been usurped by the puzzle created by Arto Inkala, which is also in the example list.

I've been looking at a new idea for measuring the difficulty of very hard puzzles - ones that can't use the standard scoring because they don't complete.

Interesting Sudokus for other reasons.

Article: The Relative Incidence of Sudoku Strategies

Article: The 17 Clue 'Proof' - my take on the January 2012 computerised exhaustive search proof.

Article: The Chaos Within Sudoku - A Richter Scale on a paper by Maria Ercsey-Ravasz and Zoltan Toroczkai at the Faculty of Physics, Babes-Bolyai University, Romania. Very interesting paper.

Article: Generalizing Sudoku Strategies by Kevin Gromley, March 2014

My response to Crooks Algorithm is here.

Article: The Relative Incidence of Sudoku Strategies

Article: The 17 Clue 'Proof' - my take on the January 2012 computerised exhaustive search proof.

Article: The Chaos Within Sudoku - A Richter Scale on a paper by Maria Ercsey-Ravasz and Zoltan Toroczkai at the Faculty of Physics, Babes-Bolyai University, Romania. Very interesting paper.

Article: Generalizing Sudoku Strategies by Kevin Gromley, March 2014

My response to Crooks Algorithm is here.

I'm pleased to include on this web site the Sudoku Song (MP3 file) by Peter Levy (official web site here). Peter wrote and recorded this song a couple of years ago and managed to capture the essence of the Sudoku craze to great acclaim.

There is a couple of definition files that I use for calibrating and testing mine. These were created a long time ago and were very popular but people may have forgotten about them. Happy to host the files here:

- Ruud's top 50,000 Most very hard
- Just17.zip 31,513 17 clue puzzles. Hard to make these

## Comments

## ... by: Hugh Pickering

## ... by: Anonymous

Cheers

- [Del]

## ... by: JAVIED AKBAR

## ... by: Wolftrakker

## ... by: Kim Uildriks

## ... by: Chucker

If C# or C++, is the source code available?

Thanks!

Source code is propriety but I have orientated this site to programmers, there are enough information to code the strategies up. That's the fun part anyway. - [Del]

## ... by: Eric

## ... by: WW

- [Del]

## ... by: BLT

All the results will highlight a strategy in the list to the right of the board. They are hyperlinked and should open in a new tab or window. There should be some documentation for everything returned by the solver so I'm interested to know where you were at this point

- [Del]

## ... by: Jonathan

But I was wondering why you do not cover the 2-String-Kite, Skyscraper or Turbot Fish strategy, which i found on many other sudoku forums, as I think that they are extremly usefull for solving sudoku puzzles.

## ... by: Brooks DeTuncq

## ... by: P Majumdar, Delhi, India.

All easy, medium and other low-difficulty level tough Sudoku puzzles cover only the very few basic simple strategies, whereas, tough/diabolic puzzles cover the remaining galaxy of advanced stategies. It is a pity!

The creator/s of all tough/diabolic Sudoku puzzles, should, provide some minimum hint, at least by naming the strategies involved towards solving the puzzle.

## ... by: jennifer antonio

## ... by: Mae

Sometimes it is fun to just take a few minute break and work and easy Sudoku. How can you encourage people to begin playing Sudoku, if you make them this tough?

## ... by: moses

## ... by: Jonathan Carr-Hopkins

Jonathan

## ... by: Geoff

I have just run across the first Five Star Sudoku where I am 3/4 finished but can not find any more hints.

Some of my friends tell me that you now need to start a 'trial & error' process ie if one cell can only be a 3 or a 5 - then pick on the 3 and see if it carries all the way through and if not, go back and pick on the 5.

I tend to think that there should always be a 'hint' to be found - is this correct?

Geoff

## ... by: Lloyd Welton

Thanks again.

## ... by: John C Raaen, Jr.